Okay, let's start by saying I am not a singing teacher, I have no formal training and the following advice is based entirely on my own personal experience and my observations while working in musical theatre and acoustic venues. If you need advice on a technical or medical problem - PLEASE see a professional.

Also, this piece of writing is intended to help people who sing in public but find it difficult and are unconfident. It's not really ideal for absolute beginners, but it may help.

The first thing to remember about singing, is that everyone can do it. Not everyone makes the same noise, but everyone possesses vocal chords. The difference is in how you treat them and how you use them. Some of the guidelines to consider are:

Regular exercise - any muscle will improve with regular exercise, your vocal chords are no exception.

Breathing - be aware of how you inhale, listen to your breath.

Warm-up - you wouldn't run the 100metres without stretching beforehand, don't expect your voice to kick in to virtuoso singing without a decent workout first.

Warm-down - if you go to all the bother of figuring out how to sing, you'd like to keep your voice for longer than 5 years. Warm that baby down when you finish a gig.

Smoking - go figure.

Exercise
Exercising your voice should do a lot of things - make you vocally stronger, more confident and more able; but it has to be one thing - fun.
The main way to improve your voice is to sing whenever you can, sing in the kitchen, sing in the bath, sing till your lover has moved out of the flat and the neighbours want to kill you. However, volume does not equal quality while exercising your voice. Regularity is the key. Sing every day. Start slowly and softly. Covers are a great way of learning about your own voice and finding your range. Play around with singing scales until you figure out just how high and low you feel comfortable with. Don't worry if neither note is quite as high or low as you wanted, extra notes will enter your comfortable range with time. The main thing to remember is, if you sing out of your range regularly it won't sound nice to the listener and it may damage your voice.

Breathing
Some people naturally know how to breath when they sing. I didn't. I knew something wasn't right, because I couldn't hold long notes or finish phrases without trailing off into a breathy rasp (mmm, nice!). I noticed other people's chests moved in a different way when they sang. I decided to check out a medical drawing of the lungs and diaphragm whereupon I discovered that because I was shallow breathing, I was only ever using about 2/5ths of my lung capacity. Trying to do deep breathing while singing felt very awkward at first, and I felt very conscious of over breathing for a while, but then it became more comfortable. I was drawing the breath right into my body and it gave me a freedom when I sang. I had almost twice as much stamina and those long notes are no longer a problem.
Listen to your breathing while you exercise. Try to control it consciously, but don't be over-conscious of it - there's the tricky part. After a while, it will become second nature.

Warm-ups
Everyone needs to warm up their voice before they sing. A warm-up can be as simple as humming for a few minutes before you go on, to a full half-hour work out involving excercises and scales. Find out what works for you. Obviously it depends on how long you'll be singing for and how difficult the material. For a bog standard half-hour acoustic set, I'd recommend a good 5/10 minutes of gentle humming, into some scales followed by some songs.

So many venues are a struggle for the vocalist because of inadequate monitoring. Am I still in tune with the guitar - who knows, because I can't hear myself sing? The great thing about warming up your vocal chords, is that it also warms up your ears. In several recording sessions I have done I've noticed that I am more often slightly sharp or flat if I have not done a full vocal warm-up beforehand.

If you are a nervous singer, a warm-up is a really good way to try and get over the shake that inevitably appears in your voice during that first song of the set. Use the warm-up to shake off the shake, as it were, and also build your confidence.

Warm-downs
This applies mainly to people who demand a lot of their voices during a set. When you sing, you contract and relax your vocal chords in all sorts of complex ways. When you push for volume while contracting and relaxing you can actually move your chord box slightly up in your throat. The best thing to do when you've just finished singing is to let your chords rest, and get some water into you. A little gentle humming and gentle massage downwards on your throat is also a good idea. Generally you only need to worry about this if you've done a particularly long set (covers bands etc.) or have had to struggle for volume. A pint of beer as soon as you walk offstage is the last thing you need, because alcohol makes your chords contract. Drinking alcohol just before, during or after a long hard gig is a surefire way to damage your voice, especially if you do it all the time, which brings me on to nodes. Nodes (or nodules) is a condition caused by misuse and/or overuse of your vocal chords and basically means you won't be able to sing for at least six months. It's definitely one to avoid. If manifests itself as small white blobs on your chords and the back of your throat, and a ragged quality to your speech and song. If you suspect you may be suffering (DON'T PANIC - it's highly unlikely), please get yourself to a doctor immediately.

Smoking
Why, oh why, oh why?
Stop. Now.
Unless you're Tom Waits.

I hope some/all of the above has been useful to some of you. Those of you who are nervous about singing, there is hope. I used to be of the opinion that my voice wasn't all that, now I really think it's something. This has come from a lot of work, a lot of advice from other people, but mainly a lot of enjoyment from singing. When you're having fun, it's hard to be nervous. Happy singing!